A hugely successful series is a hard act to follow. Those of us who loved Cressida Cowell’s brilliant How to Train Your Dragon books about Hiccup, his dragon Toothless and the Viking world they live in might have felt a little concerned when they came to an end in 2015. What would she do next?

I am pleased to report that The Wizards of Once – the first book in a new middle grade series – is terrific. It introduces us to a new fantasy world, though its roots again lie deep in a familiar mulch of history and legend. Not the Norse myths this time, but a fantasia of ancient Britain, a land of dark, mysterious forests and powerful magic.

The backstory is very simple, involving three groups of beings of both sexes, the witches (magical but evil), the wizards (magical but good) and the warriors (not magical at all). The warriors were invaders from beyond the sea, took a strong dislike to the witches, and wiped them out – or so they thought. And as they believed all magic was bad, they soon set about trying to get rid of the wizards as well.

Things are at a stalemate when the story opens. The warriors are still determined to destroy the wizards and their magic, and the wizards are equally determined to fight back. Two young characters are plunged into this toxic mix: a wizard boy called Xar, who has as yet no magic powers, and a warrior girl called Wish, who owns a banned magical object she must keep hidden at all costs.

There are rumours that some witches still survive, and Xar sets out to capture one as part of a campaign to prove himself to his disapproving father, the scary Encanzo the Enchanter. Wish has parent problems too. Her mum, Sychorax, is queen of the warrior tribe: tall, icy and deeply disappointed in her daughter. Xar and Wish meet and find themselves in an adventure that quickly spirals out of control.

What follows is a rollercoaster of suspense and surprise as they get themselves in and out of trouble. They are very different but complement each other perfectly – Xar boastful and impetuous, but with a heart of gold, Wish more thoughtful and serious, and possessed of a strong moral sense. Both are bold, brave, believable young characters children will identify with.

The book has all the familiar Cowell trademarks. The story never flags, even at moments of calm and reflection, and the writing has humour, grit and depth. She provides her own illustrations, and their scratchy style and scribbled annotations strike the right note. It all adds up to a beautifully designed hardback volume.

Several characters have names adapted or taken from The Tempest – as well as Sychorax, there’s a sprite called Ariel and a bird called Caliburn (left) – so I sense a late-Shakespearean theme of magical reconciliation being shaped for the rest of the series. But it’s clear that Xar and Wish have unwittingly unleashed real evil, so there will be plenty more alarums and excursions to come.

• Tony Bradman’s most recent book is Attack of the Vikings (Bloomsbury Education). Wizards of Once by Cressida Cowell (Hodder Children’s Books, £12.99). To order a copy for £11.04, go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99.